Far from a small magnet, its field falls as 1/r³, where r is the distance from the magnet's centre. Double the distance and B drops to (1/2)³ = 1/8. Triple it and B drops to 1/27.

Why so fast? A magnet is a dipole: an N and an S pole close together. Far away, their two fields almost cancel, and what is left shrinks faster than the 1/r² you get from a single source. That is why a magnet that grips steel at contact barely moves a compass half a metre away.